The present invention relates to a secure television system for a CATV system having "pay TV" service in which two-way communication capability between the customer and a pole mounted customer service unit is present. More specifically, the invention relates to jamming the secure channel at a customer service unit and providing a bypass system around the jammer at the customer's request.
One concept of providing "pay TV" service on a CATV system involves jamming a selected channel, thereby making the picture unusable. Access to the "pay" channel does not involve decoding at the customer's receiver, but rather bypassing the jammer circuits. Jamming is done on the cable system just prior to the customer cable "drop". One jammer unit serves several customers, with each customer controlling his individual bypass switch. The jammer unit, or customer service unit (SCU), is pole mounted and contains the jammer, bypass switch for several customers, and billing information circuits for each customer. There are several conventional techniques for achieving jamming. The pay TV channel can be filtered out with band-stop filters, or more easily, the non-pay channels only are passed through a band-pass filter, thereby rejecting the pay channel. Secondly, a phase locked oscillator signal can be added to the pay channel video carrier, causing sync and video inversion, which in turn produces a distorted picture on the customer's receiver. Also, a highly selective trap can be tuned to the pay TV video carrier frequency, producing severe amplitude and phase distortion of the pay TV signal.
However, these techniques have disadvantages which make their widespread use less than desirable. Shortcomings of the filter system are the complexity of design and alignment of an adequate filter, plus the loss of one or more channels in the cross-over region of the filter. The phase lock oscillator also requires a filter to insure that the loop is locked to the desired picture carrier and not to an adjacent channel signal. A practical system would require conversion to a low frequency IF, phase locking, and "up converting" to the carrier frequency. Additional filters would be needed to remove spurious signals falling within any other channel. The bridged T trap appears to have advantages over the other systems. It is capable of up to 60 db rejection of a single frequency, and has only about 1 db attenuation of adjacent channel signals when tuned to the desired picture carrier frequency. It is also a passive device and relatively easy to design and construct. However, the trap is temperature sensitive and requires extremely critical adjustments for alignment. For example, two high Q circuits must be simultaneously adjusted to resonance, but there is interaction between the two, thereby requiring several readjustments. Also, if the trap is not accurately adjusted to the video carrier frequency, enough signal will be passed to provide a watchable, although poor quality picture.